The city tunnel is an underground, pure S-Bahn route used by almost all services (except for the S7 service which terminates at the central station). In a short section between Mühlberg and Offenbach-Kaiserlei the South Main railway is used.
The S4 was one of the first six services of the Rhine-Main S-Bahn system. In a prior test operation it ran between Kronberg and Frankfurt Central Station. The service was then called R4 where the letter "R" stands for regional. After the opening of the Frankfurt Citytunnel the service was renamed to S4 and extended to the new Hauptwache underground station. Further extensions of the tunnel followed in 1983 (Konstablerwache) and 1990 (Ostendstraße and Lokalbahnhof) so that the Südbahnhof (South station) became the service's eastern terminal.
